This study aims to determine the optimum fattening period according to various fattening systems under controlled conditions forthe first time in lamb fattening in Turkey. The material of the study consists of 39 Tuj and 39 Hemşin genotype male weaned lambs thatare 2.5-3 months old. The Tuj and Hemşin lambs in the study have been separated into 3 equal groups through the systematic samplingmethod and subject to 90-day fattening. The amount of feed in terms of consumed dry matter for 1 kg of increase in live weightof lambs in the intensive system was calculated as 5.69 kg and 5.90 respectively for Tuj and Hemşin lambs. The optimum fatteningperiod in the intensive system for Tuj and Hemşin lambs occurred between the 70th and 84th days. The marginal cost-marginal incomeequality was not achieved throughout the fattening in the extensive and semi-intensive fattening periods for both genotypes. Thus, itis estimated that the optimum fattening period in the extensive and semi-intensive systems is over 90 days. The fattening period withthe highest marginal income was on the 70 th, 84 th, and 42 nd days respectively for the extensive, semi-intensive, and intensive systems forTuj lambs and this was on the 70 th, 28 th, and 56 th days respectively for Hemşin lambs. In conclusion, it was observed that the optimumfattening period in Tuj and Hemşin lambs was earlier in the intensive fattening period compared to other fattening periods.